1. Field of Invention
The present disclosure of invention relates to a timing controller, a liquid crystal display (“LCD”) device including the timing controller, and a driving method of the LCD device that employ an overdrive feature and in which effectiveness of overdrive is limited by a finite liquid crystal response speed.
2. Discussion of Related Technology
A liquid crystal display (“LCD”) device typically includes an LCD panel for displaying an image, a backlighting assembly for supplying light to the LCD panel, and a panel driving unit for electronically driving the LCD panel.
The LCD panel typically comprises a thin film transistors (TFT's) containing substrate having an array of thin film transistors formed thereon, a color filters substrate having an array of color filters formed thereon, and liquid crystal material interposed between the thin film transistors substrate and the color filters substrate.
In the above-configured LCD panel, optical orientation of the liquid crystals is driven by an electric field generated between a pixel-electrode provided on the thin film transistors substrate and a common electrode provided on the color filter substrate. A capacitor formed by the pixel-electrode and common electrode is charged to a field-defining voltage by passing a supplied driving voltage from a panel driving unit through the TFT and to the pixel-electrode. Transmissivity of light supplied from a backlight assembly can be controlled this way in order to display an image.
The liquid crystal material generally has intrinsic properties of viscosity and elasticity which result in a finite and relatively slow response speed to fast changing electrical pulses and in hold time driving characteristic, whereby data of a previous frame can cause a problem of old image retention during the playing of a high speed moving picture. To solve this problem, a dynamic capacitance compensation (“DCC”) method is often used for compensating for the slow response speed of liquid crystals by overdriving the liquid crystals so as to urge them to reach a target state faster, for example by applying a driving voltage pulse of higher magnitude than a target voltage to the pixel-electrode of the corresponding pixel in each image frame. FIG. 1A shows how an overdrive pulse having a magnitude denoted as overshoot is applied in frame number f(n) to a pixel in order for that pixel-electrode to thereby effectively charge the pixel-electrode to a desired Target Value by the time the time slot for driving that pixel-electrode expires. FIG. 1B shows a case where an overdriven pixel fails to reach its desired target value within the charging time of a single frame (f(n)) due to a saturation phenomenon and thus charging may have to continue in the next frame f(n+1) or beyond for the pixel-electrode to effectively charge to the desired target value.
More specifically, in one related art method of DCC driving of an LCD device, which method is represented by FIG. 1A, the magnitude of a previous frame data sample (in frame f(n−1)) is compared against the magnitude of the data sample for the current frame f(n) and the difference is determined. The amount of overshoot needed for changing state from the previous frame data state to that desired (Target) for the current frame is found for example in a look-up table (LUT) or calculated and then applied during a time slot of the current frame f(n) so as to thereby effectively reach the desired target value before the charging time slot for the current display row and frame f(n) expires. Hence, despite slow liquid crystal response speed, the pixel-electrode may often (but not always) reach the desired target value within the allotted time slot for charging that pixel-electrode (e.g., the display row charge time). While it remains the case that this overdriving method of DCC operation works when small values of overdrive are applied to the liquid crystals, there is a point of diminishing returns where the mechanism saturates as is shown in FIG. 1B and the crystals fail to respond fully to an excessive amount of applied overdrive voltage. If the overdrive voltage minus the previous pixel state exceeds a predetermined magnitude, i.e., a predetermined liquid crystal saturating step-voltage, the liquid crystals no longer respond linearly to the magnitude of the overdrive step and they therefore do not quickly reach the desired effective target value. The crystals' response instead saturates within the allotted charging time to a state corresponding to what can be called an effective liquid crystal saturation voltage.
Alternatively, it is possible to improve an apparent liquid crystal response speed by a method of stepwise partial over-driving across a plurality of frames (where f(n) of FIG. 1B might be the first partial overdrive) and then further overdriving the partially overdriven pixel in yet a next frame (f(n+1)) to thereby reach the target value in the span of two frames rather than just one. However, such a possible (but generally not practical) partial overdrive method suffers from the disadvantage that it calls for a large memory capable of storing at least two frames worth of image data, thus causing a problem that cost for the frame memories is substantially increased in case of an LCD device of high resolution. Moreover, if such a two frame partial overdrive method is used, there still occurs the problem in the case where the pixel of the previous frame is mostly black (or otherwise very dark) while the current frame is mostly white (or otherwise very bright) so that luminance in extreme changing spots of a previous frame f(n) and of a current frame f(n+1) are mutually exchanged with each other due to crosstalk on a boundary between maximum black data of a previous frame and brightest white data of a current frame and this effect is still seen as an unpleasant boundary flashing or inverting artifact by the person watching the LCD device.